1. Field of the Invention
The present invention relates to inhibiting the deposition of particulate materials in the felts of a papermaking system. More particularly, the present invention relates to a press felt conditioner which controls the deposition of polymerically flocculated particulate materials in nonacidic papermaking systems.
2. Description of the Prior Art
In a paper manufacturing process, ionically charged, relatively high molecular weight, water soluble polymers are often employed to enhance retention of cellulosic fibers, fines, and inorganic fillers. The addition of these polymers produces a cleaner process stream by reducing the solids level in the process filtrate. The high molecular weight, ionically, charged polymers control solids by adsorbing onto solid particulate surfaces in the papermaking furnish slurry and invoking charge neutralization (coagulation) and/or bridging (flocculation) mechanisms which cause the solid particles to flocculate. The flocculate can be retained by the formed mat of cellulosic fibers more easily than smaller individual particles. However, these flocculated particulate materials can be transferred from the surface of the sheet to the papermachine press felts. In the flocculated state, the particles cannot pass through the fine, porous structure of the press felt and become entrapped therein. If not controlled by adequate felt conditioning practices, these agglomerated particulate substances can severely impair the ability of the press felts to absorb water thereby requiring reduced production rates and shortening the useful life of the felts. In addition, it has been found that common polymeric retention aids can render normally effective prior art felt conditioners useless or marginally effective.
Typical polymers employed as retention aids are generally relatively high in molecular weight; for example, copolymers made from monomers such as; acrylic acid, acrylamide, dimethyl/diallyl ammonium chloride, dimethylamine, epichlorohydrin, and ethylenediamine. Typically, polymeric retention aids have average molecular weights greater than 1,000,000 and can range up to around 20,000,000 for anionically charged polymers and up to 15,000,000 for cationically charged polymers. These ionically charged polymers adsorb onto the various solid particulate surfaces within the papermaking furnish causing flocculation.
In neutral and alkaline papermaking (pH from approximately 6-8.5) the use of polymeric retention aids is particularly critical for efficient operation. Without the use of such retention aids, common system additives such as cellulose-reactive sizes can cycle up in the process system and hydrolyze causing system upsets. As a result, in most neutral and alkaline papermaking systems most of the particulate contaminants that are transferred from the sheet to the press felt are flocculated. Such flocculated particles are relatively unresponsive to typical felt conditioning treatments.
Analysis of used press felts from neutral and alkaline papermaking systems reveal a number of polymerically flocculated materials. Significant quantities of inorganic fillers such as; calcium carbonate, clay, and titanium dioxide alone or in association with particles of latex coating binders such as, polyvinyl acetate or styrene-butadiene rubber are found. Other types of contaminants that can be associated with the agglomerates, in significantly lower quantities, can include starches, natural wood pitch (fatty esters, fatty and resin acids/salts) cellulosic fiber fines, microbiological contaminants and absorbants, such as talc or bentonite, oil-based defoamers and insoluble metal hydroxides. All of these contaminants can be present to some degree based on a variety of factors, such as, the wood fiber and water sources, time of year, grade of material produced, type and quantity of system additives, pulp production methods and equipment designs and capacities.
Processes to inhibit contamination deposition in papermaking felts are known in the art. U.S. Pat. No. 4,715,931, Schellhamer et al. discloses a process for inhibiting aluminum hydroxide deposition in papermaking systems which comprises adding to the felts a hydroxylated carboxylic acid. The use of the carboxylic acid in combination with surfactants, such as octyl phenol ethoxylates, nonyl phenol ethoxylates and others listed therein inhibit aluminum hydroxide deposition and associated organic contaminants.
In addition to the control of inorganic contaminants, pitch formation is of concern in papermaking systems. Filler materials can become trapped within the organic matrix formed by pitch coalescence compounding deposition problems. Methods of controlling the pitch deposition formation are known in the art, U.S. Pat. No. 4,184,912, Payton, discloses a method of preventing pitch formation by dispersing and emulsifying pitch particles in the pulp furnish to an exceptionally fine state and uniformally distributing the particles throughout the finished paper. The pitch deposition is controlled by the addition of a three component formulation comprising a nonionic surfactant plus an anionic surfactant and a low molecular weight anionic polymer. The three component mixture is added to the papermaking pulp system at a point prior to where pitch deposits normally form. In U.S. Pat. No. 3,992,249, Farley, a process for inhibiting pitch deposition is disclosed wherein the pulp is washed with an aqueous solution of anionic polymer having between 25 to 85 mole percent hydrophobic-oleophilic linkages and 15 to 70 mole percent hydrophilic acid linkages to complex with the pitch. The pitch-polymer complex is washed away with water.
U.S. Pat. No. 3,873,417, Otrhalek et al., discloses a pitch and pigment dispersant which comprises a neutralized solution of polymer prepared by free radical polymerization of an alpha, beta unsaturated acid with an alkyl ester and an allyl alcohol.